Electronic packaging is essentially the final step in the process of fabricating an integrated circuit (IC) or “chip.” In the packaging step, the semiconductor die is mounted on a leadframe, and electrical connections are made between the die and the leads of the leadframe, before encapsulating the assembly in the component body. The leadframe is typically made from a thin sheet of metal by stamping or etching. The majority of IC packages use a method known as wirebonding to make the electrical connections between the die and the leads. In wirebonding, one end of a very fine gold or aluminum bondwire (on the order of 10-100 micrometers in diameter) is attached to a bondpad on the die, and the other end is attached to an end of a lead. The end of the lead to which the bondwire is attached is sometimes referred to as a bondfinger and sometimes as a lead tip. The bondwire ends are typically attached by applying some combination of pressure and heat. The resulting weld-like or solder-like attachment or bond is commonly referred to as a wirebond. The other end of the lead extends externally to the package (component body) for connection to other circuitry when the IC is used as a circuit component. This external portion of the lead may have a stepped shape or other features to facilitate mounting it to a circuit board, but such features are not relevant to the present invention.
Many common ICs, including many application-specific integrated circuits (ASICs), are in the form of a package known as a Quad Flat Pack (QFP). As illustrated in FIG. 1, a QFP package 100 has four sides with leads 102 extending from the component body 104 on all four sides. As illustrated in FIG. 2, inside a QFP package 100 the electrical connections between the die 106 and the leads 102 are made by wirebonding with bondwires 108. (Some leads 102 and other grouped elements are not shown individually for purposes of clarity, and their omission is indicated collectively by an ellipsis (“ . . . ”) symbol.) Typically, one wirebond (i.e., the weld-like attachment or bond) connects or attaches one bondpad 110 to one lead 102. In general, the number of bondpads 110 dictates the number of corresponding QFP leads 102. Nevertheless, in instances in which adjacent or proximate bondpads are intended to carry the same electrical signal (e.g., device ground and power), more than one bondpad may be wirebonded to a single lead. Typically, there are numerous electrically common bondpads that lie along each of the four sides of the die, such as device ground (“VSS”) and core power (“VDD”).
In some packages, ground bondpads are wirebonded to diepaddle 112, which is a metal plate-like portion of the leadframe underneath the die that physically supports the die and acts as an electrical ground plane. (The leadframe is the entire stamped or etched area comprising all leads 102.) The diepaddle 112 effectively combines and consolidates the ground connections from the die and redistributes ground to a number of package lead tips. A similar structure, sometimes referred to as an interposer (not shown), can be used for distributing core power. Such structures add significant manufacturing cost and increase the size and weight of the package.
The more leads in a QFP package, the greater its size, weight, cost and complexity. Thus, it would be advantageous to minimize the number of package leads. In some instances, a package for which a component manufacturer or customer indicates a preference has too few leads to accommodate the number of bondpads of the die to be packaged. In such an instance, a larger, more complex, less economical package must be selected. Note in the QFP package 100 of FIG. 2 that a few bondpads 110a, 110b and 110c cannot be accommodated and thus have been left unattached. While in this exemplary QFP package 100 it may be that bondpads 110a, 100b and 100c are not utilized by the die 106, i.e., carry no signal in operation, and thus are not to be accommodated by any leads 102, it would be desirable to provide the option of accommodating them.
It would be desirable to provide an IC package that maximizes the number of bondpads that a given number of leads can accommodate. The present invention addresses the above-described problems and deficiencies and others in the manner described below.